A bearing is required to have an excellent rolling contact fatigue life, and in recent years, there has been a problem of shortened rolling contact fatigue life of a bearing made of conventional bearing steel (JIS-SUJ2) for use in, for example, automobiles, windmills, and industrial machine. In view of this, various studies have been made on the improvement of rolling contact fatigue life. An example of the bearing member includes a bearing for use as an alternator of an automobile.
It has already been known that the rolling contact fatigue fracture, which determines the rolling contact fatigue life of a bearing, is caused by microstructural change called white structure generated right on the rolling contact orbit of the bearing, resulting in a crack generated and extended in the bearing.
Here, the aforementioned white structure is categorized into the following three types:
(1) white structure (hereinafter, also referred to as WEA), which is randomly generated without having any specific orientation relationship relative to the rolling contact orbit;
(2) white structure (butterfly), which is generated on the periphery of a non-metal inclusion in a direction at 45°; and
(3) white structure (white band), which has orientations of about 80° and about 30° relative to the rolling contact orbit.
In particular, the generation of WEA leads to a bearing fatigue fracture in a shorter time before reaching expected rating life of the bearing, and thus WEA should be most urgently dealt with.
The WEA is considered to be generated, as described in Patent Literature 1 (PTL 1), through the following mechanism. That is, a lubricant such as grease or a lubricant oil used in the bearing and water penetrating into the bearing are decomposed through a tribochemical reaction to generate hydrogen, and the hydrogen thus generated penetrates into the steel and is accumulated therein so as to accelerate the generation of WEA.
As examples of measures to be taken to suppress the generation of WEA, Patent Literature 2 (PTL 2) discloses a method of increasing Cr content and N content to thereby increase the amount of retained austenite, and Patent Literature 3 (PTL 3) discloses a method of adding Ni alone or of simultaneously adding Ni and Mo together.